1. Field of the Invention
The present invention relates to a conveyance control mechanism for a printer provided with an image forming unit configured to form an image on a surface of a sheet conveyed on a conveyance route, and relates to a conveyance control method.
2. Description of the Related Art
In general, in order to record an image on a sheet, an inkjet-type image forming apparatus constantly needs to maintain an interval between a sheet and multiple recording nozzles of a printing head at an optimum distance for recording.
A sheet may often curl above a sheet conveyance unit. If a sheet curls considerably upward, the interval between the sheet and a recording head deviates from a preset interval, thereby causing a failure in recording on the sheet according to the settings. In some other cases, the sheet may strike against the recording head and may damage the recording head as a consequence.
Meanwhile, image forming processes at higher speed has been demanded for image forming apparatuses in recent years. To meet this demand, an image forming apparatus continuously feeds numerous sheets from a paper feeder at short intervals when continuously performing image forming processes on the numerous sheets.
In such an image forming apparatus configured to perform an image forming process at high speed, a process to stop a whole of printing and conveyance operations is carried out when a sheet is jammed somewhere on a conveyance route. Accordingly, the apparatus stops operating while having numerous sheets on the conveyance route. Removal of all the sheets on the conveyance route at the time of a jam release operation for a jammed sheet complicates the jam release operation, and also causes a waste of recourses because numerous usable sheets are wastefully discarded.
To address this situation, Japanese Patent Application Publication No. 2002-137849 discloses a technique in which, when a sheet is jammed somewhere on a conveyance route, all the completely-recorded sheets that are being conveyed on a downstream side of the jammed sheet in a conveyance direction are discharged.
Here, a sheet in conveyance may be curled not only at its front end but also at its tail end. In this case, if a curl of the sheet is detected at the front end thereof, the printing is not performed on the sheet. Instead, if a curl of the sheet is detected at the tail end thereof, the printing has already been started on the sheet from the front end in some cases. In the latter case, if a belt platen continues the operation to convey the sheet, a recording head may be damaged.
Regardless of whether the curl occurs at the front end or at the tail end of the sheet, however, all the sheets are transmitted through the conveyance route on the downstream side in the conveyance direction and discharged onto a discharge tray according to the technique disclosed in Japanese Patent Application Publication No. 2002-137849. Therefore, the curled sheet also passes through a belt platen unit and thereby may damage a recording head. Moreover, since sheets correctly printed and sheets printed only partly are discharged onto the same discharge tray, a user needs to carry out a cumbersome operation later for removing the sheets printed only partly. Such an operation puts a heavy burden on the user.
Meanwhile, as a method of automatically reversing print sheets in a printer configured to perform both side printing, there has been widely used a method in which the printer is provided with a switchback route in the course of a route for circulating print sheets, and thereby is configured to reverse the front and back surfaces of a print sheet by performing switchback of the print sheet on the switchback route.
However, a print sheet may be retained in the switchback mechanism at the time of an emergency stop due to occurrence of a sheet jam. In this case, the above-described technique disclosed in Japanese Patent Application Publication No. 2002-137849 does not allow the print sheet to be completely discharged from the switchback route. Therefore, the user has to remove the print sheet. In addition, due to complexity of the switchback mechanism, the operation to remove the jammed sheet is even more complicated and leads to an increase of the burden on the user.